Estrogen exposure has been linked to the formation of prolactin-secreting adenomas (prolactinomas), the most prevalent form of pituitary tumors in humans. The etiology of prolactinomas is not well understood. Preliminary data indicate that transforming growth factor-beta3 (TGF-beta3) may regulate lactotropic cell proliferation and thereby the development of prolactinomas. In addition, these data indicate a novel mechanism of action for TGF-beta3 via a pracrine interaction with folliculostellate (FS) cells and lactotropes. FS cells are agranular cells of the anterior pituitary generally thought to carry out supportive functions. The interaction between FS cells and lactotropes in mediating TGF-beta3 action suggests a unique regulatory role for FS cells. The objective of this proposal is to elucidate the mechanism by which TGF-beta3 functions in estrogen-induced lactotropic cell proliferation and identify the involvement of FS cells. Three specific aims are proposed for addressing this interest. SPECIFIC AIM 1 is to determine the site(s) of action of TGF-beta3 by utilizing afinity-labeling and immunoprecipitation techniques to identify if TGF-beta3 binding sites exist on lactotropes and/or FS cells. SPECIFIC AIM 2 is to evaluate if the interaction between FS cells and lactotropes involves secretion of a soluble factor by using cell purification, separation and condition media techniques, or if it involves gap-junctional communication by using fluorescent dye transfer methods. SPECIFIC AIM 3 is to identify the soluble factor or factors released by the FS cells by methods including radioimmunoassays, immunoneutralization and antisense RNA block procedures. The proposed research will provide significant information concerning the newly identified pituitary TGF-beta3 system and the involvement of this system in the regulation of estrogen mitotic action on lactotropes. Such knowledge should be important in understanding prolactinoma because increased proliferation is an early step in cell transformation.